We have an Exchange 2019 Hybrid server to remote mail-enable our AD users and send outbound to our mailboxes from certain legacy apps. Though our users are all AD-bound, we create some mailboxes (e.g. shared ones) directly in EOL/M365 (aka no AD object).
The hybrid server reliably sends mail to all of our AD-bound remote mailboxes, but as soon as we change the recipient to a shared mailbox created in EOL directly (no AD object), we get something like this:
<550 4.4.7 QUEUE.Expired; message expired in unreachable destination queue. Reason: A matching connector cannot be found to route the external recipient>
Also, for what it's worth, I noticed in the message tracking logs that when we address to "[email protected]" the successful deliveries do a routing source resolve to "[email protected]". Our shared mailboxes are created in EOL directly and would only have "[email protected]" by default.
As I was writing this up, I tried adding a secondary SMTP of "[email protected]" through EOL, and sending another test message, but the hybrid host still considers the shared mailbox as an external recipient.
If anyone has experienced this and has a suggestion, I'd be grateful. Thanks!